Field of the Invention
The invention relates to a semiconductor manufacturing technology and more particularly to a transistor structure.
Description of Related Art
In the integrated circuit of a semiconductor, the field effect transistor is a major component. A field effect transistor is basically the structure of the metal-oxide-semiconductor (MOS), including a gate structure and a source region and a drain region located at two sides of the gate structure and on the substrate. Besides, a gate insulation layer is disposed between the substrate and the gate structure. Generally, a thickness of gate insulation layer determines a threshold voltage when the transistor starts operating. Since the gate insulation layer is formed by performing a thermal oxidation process to the substrate, the thickness of the gate insulation layer on the edge region thereof is thinner, and the threshold voltage of the transistor is influenced.
Besides, if the thickness of the gate insulation layer is excessively thin and the operational voltage is excessively high, for example, is within an operating range of medium voltage (MV) or high voltage (HV), a leakage may occur.
The above problems occur more significantly when the transistor has a smaller size. Therefore, in the design and manufacturing processes, factors affecting the quality of transistors needs to be eliminated to as few as possible.